When burning fossil fuels to produce energy, one typically uses a high temperature combustion process in the presence of air. Unfortunately, this type of process produces both nitrogen oxides (NOx), which are well-known pollutants, and other components that are harmful to health or the environment, such as carbon monoxide and unburned hydrocarbons. Thus, it is important to remove these materials prior to their release into the environment.
There have been many investigations into methods that allow for the removal of these substances. Two methods that are known are combustion modifications and adsorption techniques. Unfortunately, each of these has its disadvantage. The former allows for only limited maximum removal of NOx, and the latter has limited capacity.
A third method for addressing the problem of noxious exhaust gases is catalytic removal, which by comparison, is extremely effective in removing large proportions of unwanted exhaust components and is capable of treating very large volumes of exit gases for long periods of time. In order to effect the reduction of NOx in exhaust gases through catalytic reduction processes, it is necessary either to introduce a reducing agent, such as ammonia, and/or to use the unburned hydrocarbons present in the waste gas effluent. The latter may be more desirable in many applications because it facilitates the combined elimination of two undesirable exhaust components, nitrogen oxides and hydrocarbons, and avoids the introduction of another component into the gas stream.
Although catalytic removal of exhaust gases is common in many industries, the method has unfortunately not been sufficiently optimized. As an example, it is well known that noble metal containing catalysts are effective in the reduction of NOx emissions using either ammonia or hydrocarbons as reducing agents. However, noble metal containing catalysts often have a significant activity for oxidation of sulfur dioxide to sulfur trioxide, and it is well understood that sulfur-containing compounds both adversely affect the performance of noble metal containing catalysts and act as irreversible poisons. It is also known in the art that certain metal oxides used as catalyst supports for noble metals, such as aluminum oxide, are adversely affected by sulfur oxide attack. Thus, many strategies for developing a catalyst are impeded by the undesirable effects of sulfur-based compounds.
As persons skilled in the art are aware, certain work has been done to address the negative effects of sulfur-based compounds. For example, titanium dioxide, particularly that produced in the sulfate process and which contains residual sulfate, has been investigated because it is now well known to resist sulfate poisoning. Further, it has been reported that the performance of vanadium pentoxide, tungsten trioxide and molybdenum trioxide can be improved by supporting them on titanium dioxide. Still further, increased benefit has been shown to be achievable by forming solid solutions of molybdenum or tungsten oxides with vanadium oxide rather than having them exist independently on the surface or exhibiting distinct phase boundaries between oxides.
However, despite the knowledge of the potential for the use of titania supported vanadium oxide catalysts, none of the known technologies satisfactorily optimize NOx treatment while avoiding undesirable oxidation of sulfur dioxide. The present invention is directed to developing an improved catalyst to address these issues.